How do modified nanoclays adversely affect aquatic species relative to natural nanoclay?

Nanoclays represent a large class of modified nanomaterials (NMs) (i.e. nanoscale particles from 1 to 100 nm) that has received great attention from the scientific and industrial communities. This material has been widely incorporated into conventional polymers in order to improve their barrier properties, thermal and mechanical resistance, and to reduce their costs. Currently, some nanoclays are also useful in water treatment and pollution control for removing toxic chemicals from water supplies. However, due to the increased frequency of industrial and environmental applications of the modified nanoclays, their release into the environment will be inevitable in the next decade, especially in the aquatic ecosystems. Thus, many concerns have been raised about the lack of information and potential adverse effects of modified nanoclays relative to natural nanoclay already present in the environment. This dissertation has investigated the physicochemical characterization of natural nanoclay (Na+ montmorillonite) and two modified nanoclays (Cloisite[registered] 30B and Novaclay[TM]), and their potential toxicity on algal population growth of Chlamydomonas reinhardtii, on survivorship and body growth of Daphnia magna and Chironomus dilutus, and on survivorship, body condition, and liver tissues of Gambusia holbrooki. This study found that particle size of nanoclays was dependent on the nanoclay concentration in solution, which resulted in the formation of agglomerated particles at lower concentrations and deagglomerated particles at higher concentrations due to particle-particle collisions. In addition, surface charge analysis showed that Novaclay[TM] is more stable than natural nanoclay and Cloisite[registered] 30B, making Novaclay[TM] more likely to remain as single particles rather than agglomerates in solution. Although, natural nanoclay and Novaclay [TM] retained their platelet-type shape in aqueous medium, Cloisite[registered] 30B transformed from a platelet-type shape in dry powder form to spherical particles when in solution. This study also found that natural nanoclay and modified nanoclays had important implications to aquatic life; however, there were differences in their toxicity due to nanoclay composition, concentration in media, particle size and shape, surface charge, and exposure time. In general, the toxicity of the three types of nanoclays to aquatic species rank as follows: Cloisite[registered] 30B> Novaclay[TM]> natural nanoclay. Cloisite[registered] 30B adversely affected more kinds of organisms than other nanoclay types. Cloisite[registered] 30B, even at low concentrations, reduced growth rate of C. reinhardtii, D. magna survival, body growth of C. dilutus and caused liver tissue damage to G. holbrooki. Novaclay[TM] and natural nanoclay also induced histopathological changes on liver tissues of G. holbrooki at very low concentration. Conversely, NovaclayTM only reduced the survivorship of D. magna during chronic exposure at low and high concentrations, while natural nanoclay caused a decline in daphnid survival only at high concentration with acute exposure. None of the nanoclays affected the survivorship of C. dilutus or G. holbrooki during the duration of our study. We also found little effect of natural nanoclay and Novaclay[TM] on the body growth of D. magna and we were unable to assess the effects of Cloisite[registered] 30B on the body growth of daphnids at higher concentrations, because all organisms died when exposed to Cloisite[registered] 30B. In addition, none of nanoclays caused any effects on body condition of G. holbrooki after 14 days of exposure. The higher toxic effects of Cloisite[registered] 30B may be associated with the presence of quaternary ammonium compounds in its composition, which may cause oxidative stress in biological systems. While, the toxicity of Novaclay[TM] and natural nanoclay is probably due to their high stability in aqueous medium that makes them more available for pelagic species (e.g.; daphnids and mosquito fish). Thus, we should be careful about the kinds of modified nanoclays that we introduce into aquatic environments, since they pose a threat to aquatic organisms and ecosystems processes.